Supply and demand are two sides to the same coin.
Sales and marketing generate demand. Engineering and manufacturing generates supply. Both must exist for a product to be meaningful in the market.
The labor theory of value, you see, is bunk. The product is worth only what the buyer will pay for it. And much of the time, a salescritter can convince someone to pay so much more for a product above cost that the difference pays for the salesperson's work, and still leaves some left over for R&D, or other nice things.
Don't get me wrong. I still hate smarmy, schmoozing salesfolk with the passion of one ten-thousandth of a sun. But they aren't entirely useless. They do add value to a company in their own right. It is as a value multiplier, though. There still has to be some value added before you can multiply it.